JP2010137231A - Butt-welded pipe having good workability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a butt-welded pipe having good workability, specifically a butt-welded pipe having good workability which does not cause cracks at a joining part when subjected to strong working such as flaring. <P>SOLUTION: The ratio of a length 11 in the thickness direction of the joining part to a pipe thickness 12 in the butt-welded pipe 8 is made to be 1.00 or more. The ratio of a maximum thickness 14 of a bead part to the pipe thickness 12 is 1.010 or more. Butt welding is carried out by allowing an upset ratio during butt welding to be 1.0% or more. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a forged welded tube having good workability, and more particularly to a forged welded tube having good workability in which cracking does not occur in a joint even when subjected to strong working such as flaring.

In recent years, pipes that have a joint portion thereof are increasingly subjected to strong processing on the end of the pipe, such as flare processing. Therefore, an electric resistance welded tube is applied as having the ability to withstand this strong work, but since an electric resistance welded tube is expensive, an inexpensive forged welded tube is applied.
Conventional forged pipes are said to have insufficient performance to be applied to applications where flaring is applied because the strength of the joint is low and cracking tends to occur at the joint when it is hard-worked. .

  For example, as shown in FIG. 1, the forged pipe is manufactured by forming a slit steel strip (also referred to as a plate) 2 with an edge forming machine 4 and heating it with a heating furnace 5. In the case of continuously forming into a tubular shape, oxygen or air is blown with nozzle 7 to both ends in the circumferential direction of the tubular molded body, that is, the abutting end portion, the abutting end portion is heated by oxidation heat, forged and joined. In some cases, the tube 8 is finished by drawing. In addition, although not shown in figure, edge shaping | molding may be carried out after cutting the edge of the slit steel strip.

  The manufactured welded tube has oxides and the like easily remaining in the joint part, and as shown in FIG. 2, streaky wrinkles are generated on the outer surface and inner surface of the joint part. Cracks occurred in the joints during strong processing. The streaks 13 on the outer surface side of the joint portion are formed by slitting the steel strip and the sagging generated at the end portion remaining during forging. In addition, the inner side streak 9 has a bead portion 15 that is raised at the time of joining to form a bead portion 15, and this valley is a streak.

Therefore, conventionally, as shown in Patent Documents 1 to 3, the outer surface side muscle depth, the inner surface side bead height, the inner surface side muscle depth, the inclusions in the joints, and the like are within a specific range. By regulating, forged pipes that improve the strength of the joints have been provided.
JP 2007-152430 JP 10-263846 JP 4-270009

  However, it has been understood by the inventors that the outer surface side muscle depth, the inner surface side bead height, the inner surface side muscle depth, and the inclusions in the joints are within a specific range. In many cases, the strength of the joint portion of the tube cannot be sufficiently improved, and the joint portion is often cracked by flare processing. In other words, the effect of the outer surface side, inner surface bead height, and inner surface muscle depth on the strength of the joint is an apparent phenomenon, and the conventional method using such an apparent phenomenon is true. The phenomenon was not grasped and it was not an essential solution.

In view of the above situation, the present invention was created in order to provide a forged welded tube that does not break from the joint even when subjected to strong processing such as flare processing. It is as follows.
(1) A forged welded tube having good workability, wherein the ratio of the length in the thickness direction of the joint in the forged welded tube to the tube thickness is 1.00 or more.
(2) The forged welded tube having good workability as set forth in (1), wherein the ratio of the maximum bead thickness to the tube thickness is 1.010 or more.
(3) The forged pipe having good workability as set forth in (1) or (2), wherein the upset rate is 1.0% or more and forged.

  ADVANTAGE OF THE INVENTION According to this invention, even if it uses for strong processes, such as a flare process, the forgeable pipe | tube with favorable workability which does not generate | occur | produce a crack in a junction part can be provided.

  Conventional forged welded pipes were prone to cracking at the joint when subjected to strong processing such as flaring. This was thought to be due to the fact that inclusions such as oxides were likely to remain in the joints during the manufacture of the forged welded tube, and streak was generated on the outer and inner surfaces of the joints. In other words, it has been thought that if oxides or the like remain in the joint, it is likely to become a starting point of cracking, and streaky wrinkles on the outer surface and inner surface of the joint portion are also likely to start cracking.

Therefore, based on the cause of cracks in these joints, as described in Patent Document 1 to Patent Document 3, in the past, forged joint pipes in which the inclusions in the joints have been reduced and the streaks on the outer surface or the inner surface have been reduced. However, sufficient strength of the joint portion could not be obtained only by making them appropriate.
Therefore, the inventors paid attention to the length 11 in the thickness direction of the joint 10 shown in FIG. 2 and FIG. When observing in detail the conventional welded pipe having a low joint strength shown in FIG. 2, the thickness direction length 11 of the joint portion 10 is short, and it acts strongly in the pipe circumferential direction in strong processing such as flare processing. It has been found that the strength of the joint that overcomes the tension is insufficient.

  The strength of the joint will be described in more detail. When the length 11 in the thickness direction of the joint 10 is shorter than the tube thickness 12, the joint strength is reduced, and when the thickness is equal to or longer than the tube thickness 12, the joint strength is improved. That is why. That is, in strong processing such as flare processing, the tube end portion and the periphery thereof are expanded in the circumferential direction while being expanded. At that time, excessive tension acts in the circumferential direction at the pipe end and the periphery. This tension tends to concentrate on the thin part of the pipe, so if the thickness of the joint is thin, that is, if the length of the joint in the thickness direction is shorter than the pipe thickness, the stress will be concentrated on the joint. And it becomes easy to break.

In the past, it was not possible to focus on the stress concentration due to this excessive tension, so the inclusions in the joints and the internal and external streaks were considered to be the starting point of cracking, so we took apparent measures and took measures accordingly. As a result, sufficient strength could not be obtained at the joint.
In the present invention, as shown in FIG. 3, the thickness direction length 11 of the joint portion 10 is made longer than the tube thickness 12, that is, the ratio of the joint thickness direction length 11 to the tube thickness 12 is 1.00 or more. By doing so, stress concentration due to excessive tension in strong processing such as flare processing can be alleviated, and thereby cracking of the joint can be prevented.

Here, the pipe wall thickness 12 may be the average wall thickness in the circumferential direction of the forged pipe, or may be the wall thickness of the portion located on the opposite side of the pipe joint, and the wall thickness is substantially equal around the joint. As a specific position, for example, the thickness of the position separated from the joint by a distance equivalent to the length in the joint thickness direction, the thickness averaged in the range of 1/4 in the tube circumferential direction across the joint, etc. Also good.
The inventors further studied a method for improving the strength of the joint. The stress concentration that leads to cracking of the joint acts not only on the interface of the joint but also on the periphery thereof. In addition, oxygen or air is blown to the abutting end during forging, so the temperature around the abutting end, which becomes the bead after the forging, rises and some of the metal elements tend to diffuse. Unlike, the deformation resistance after manufacture changes and in some cases is somewhat lower. As a result, it may be susceptible to cracking under the influence of excessive tension of strong processing such as flare processing.

Therefore, attention is paid not only to the thickness direction length 11 of the joint portion 10 shown in FIG. 3, but also to the bead portion 15 generated in the periphery thereof. The bead portion 15 is a portion where the abutting end portion is raised at the time of joining. In the case of a forged welded tube, the bead portion 15 tends to rise mainly on the inner surface side, but slightly rises on the outer surface side.
If the bulge of the bead portion 15 is large, the thickness increases, so that the tension per unit area of the cross section is reduced, the stress concentration is relaxed, and the crack is reduced even if the deformation resistance is lower than other parts of the pipe. Can be prevented.

Therefore, by increasing the thickness of not only the joint 10 but also the bead 15 around it, the stress concentration around the joint in strong processing such as flaring can be alleviated and cracking can be sufficiently prevented. . As a result of quantitative studies by the present inventors, it has been found that the joint strength can be improved when the ratio of the maximum thickness 14 of the bead portion 15 to the tube thickness 12 is 1.010 or more.
In addition, the present inventors, together with the increase in the ratio of the junction thickness direction length 11 to the tube thickness 12 or the increase in the ratio of the bead portion maximum thickness 14 to the tube thickness 12, It has been found that the strength of the joint is improved by increasing. Here, the upset rate is a reduction ratio of the length in the pipe circumferential direction after forging with respect to the width of the steel strip on the entry side. Mainly, the circumferential length of the outer circumferential portion of the tube is often used as the circumferential length of the tube, but this is not particular. According to the study by the present inventors, it was found that the joint strength can be improved by setting the upset ratio to 1.0% or more. By increasing the upset rate to 1.0% or more, a large compressive force can be applied to the joint from the abutting direction, and the strength of the joint can be improved. In order to increase the upset rate, it is not sufficient to simply press the abutting end at the time of forging, and it is possible to further increase the temperature at the abutting end at the time of forging to reduce deformation resistance. is required. For example, normally, air is blown to the heated abutting end and the temperature is increased by oxidation heat, but by blowing oxygen instead of air, the oxidation of the abutting end can be further promoted to increase the temperature. Since it can be further increased, the upset rate can be improved.

  Even if the upset rate is constant, the thickness direction length of the joint portion and the maximum thickness of the bead portion change by changing the temperature distribution around the abutting end portion during forging. In order to change the temperature distribution around this abutting end, the temperature distribution in the width direction of the steel strip on the inlet side, the amount of air or oxygen sprayed during forging, and induction heating means are installed on the inlet side of the forming forging machine. These may be controlled such as the temperature at which the end portion is heated. By changing the temperature distribution around the abutting end portion, the deformation resistance distribution is changed, whereby the thickness direction length of the joint portion and the maximum thickness of the bead portion are changed.

  Therefore, in addition to increasing the thickness direction length of these joints and the maximum thickness of the bead part, if the upset rate is increased, compressive force can be applied from the abutting direction, and the strength of the joint part can be further improved. It is.

Hereinafter, the present invention will be described more specifically with reference to examples.
A forged pipe was manufactured in the manufacturing process shown in FIG. That is, the steel strip 1 is formed into a sheet width end portion with an edge forming machine 4, heated to a temperature below the melting point with a heating furnace 5, and formed into a tubular shape with a forming forge machine 6, while a nozzle 7 is formed at the abutting end portion. Then, air or oxygen was blown and then forged and joined, and drawn and rolled to produce a forged tube.

Manufacturing conditions such as upset rate in the above manufacturing process, ratio of the thickness direction length 11 to the tube thickness 12 of the joint portion 10 of the manufactured forged pipe shown in FIGS. 2 and 3, and the maximum thickness of the bead portion Table 1 shows the ratio between 14 and the tube thickness 12 and the presence or absence of cracks in the joint after flaring.
In this example, the upset rate is the ratio of the decrease in the circumferential length (L) of the outer periphery of the pipe after forging with respect to the width (W) of the incoming steel strip (upset rate = (WL) / W × 100 (%)).

  As is clear from Table 1, in the comparative example, the strength of the joint portion was insufficient and cracking occurred, whereas in the inventive example, no cracking of the joint portion occurred.

  The forged welded tube manufactured according to the present invention has a good joint strength, and even when subjected to strong processing such as flare processing, the joint does not crack and has remarkably good performance. It can withstand strict demands on the pipes, and its effect is extremely great.

Schematic showing an example of the manufacturing process of forged pipes Schematic showing the cross-sectional shape around the joint of a conventional welded tube Schematic showing the cross-sectional shape around the joint of the forged pipe of the present invention

Explanation of symbols

1 Coiler 2 Slit steel strip (plate)
3 Looper 4 Edge forming machine 5 Heating furnace 6 Forming and forging machine 7 Nozzle 8 Forging and connecting tube 9 Streaks generated on the inner surface side
10 joints
11 Thickness direction length of joint
12 Tube thickness
13 Streaks on the outer surface
14 Maximum bead thickness
15 Bead section

Claims (3)

  Forged welded tube with good workability, characterized in that the ratio of the length in the thickness direction of the joint in the welded tube to the tube thickness is 1.00 or more.   The forged pipe having good workability according to claim 1, wherein the ratio of the maximum thickness of the bead portion to the tube thickness is 1.010 or more.   The forged welded tube having good workability according to claim 1 or 2, characterized by being forged with an upset ratio of 1.0% or more.

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